Arrangement for hydrocarbon extraction in oil wells

ABSTRACT

An extraction arrangement for the extraction of hydrocarbons that allows reducing related costs by simplifying the structure, reducing the number of parts used in this type of arrangement, and consequently reducing the associated risks and facilitating both operation and maintenance tasks, also optimizing the extraction capacity of the pump by not having leaks in seals because the latter are not necessary as it is a fixed tube.

STATE OF THE ART OF THE INVENTION Field of the Invention

The present invention relates to an arrangement used in oil field wells for the extraction of hydrocarbons, and more particularly relates to an arrangement for hydrocarbon extraction comprising a pipe or suction tube arranged in a fixed position in the wellhead, making only one collection container go down and up through the well in order to reduce the number of seals, gaskets, joints, airtight chambers and similar devices used in this type of arrangement, and in turn, making the extractor arrangement of the invention much more practical for its maintenance as well as for its operation, significantly reducing related costs.

Description of Prior Art

In the oil industry, more particularly in exploration, the so-called low-pressure or semi-exhausted wells are known, which are wells that over time and their exploitation reach the point of containing an amount of oil and a gas pressure that impose the use of special equipment and techniques to be able to extract these products still remaining in the well.

One of the solutions used in relation to the problem of extracting the remaining gas and oil is disclosed in U.S. Pat. No. 7,866,381 belonging to the same owner of the present invention. According to FIGS. 1 and 2a-2c related to the prior art of said US Patent, the arrangement for the extraction of hydrocarbons comprises an extraction assembly made up of a container, hose or fluid collector tube 2 and a suction tube 1, and housed inside a casing pipe 3, normally the well casing. The suction tube 1 extends along the hose or collector tube 2, which has an open upper end 4, through which the upper section 8 of the suction tube 1 passes, at whose end it is fixed to a traction cable 11 of the aforementioned assembly. The collector tube 2 is welded to the suction tube 1. In turn, the hose or collector tube 2 has a lower end 5 closed by means of a one-way check valve 6 that allows fluid to enter the space between the hose or collector tube 2 and said suction tube 1. The upper section 8 that projects beyond the upper end 4 of the collector hose 2 has a fluid discharge hole 10 formed laterally. The operation of the device is based on the fact that the “hose-collector tube 2/suction tube 1” assembly is lowered into the mass of oil and once the oil has filled the container or hose 2, the assembly is raised by the action of the traction cable 11 until it reaches the end of its stroke. There, said discharge hole 10 is positioned in a sealed chamber 16 of the well head defined between a pair of sealing rings 12-13, so as to allow the discharge of the fluid by means of the suction of a pump 18 and, when there is internal pressure in the well, with the help of the gas pressure that exists inside defined by the jacket or casing.

U.S. Pat. No. 6,464,012 discloses an oil extraction system that has a cylindrical container, suitable for being inserted inside the casing. The container is coupled to a cable to lower it into the oil, which enters through the lower part of the container, which has a sphere that is seated on a hole in said container, in order to allow the entry but not the exit of the oil. The cylindrical container at its top has a hole where a pressurized air supply line is seated when the container is moved up and positioned against an air injection nozzle. The fluid that is in the container moves through a suction pipe by virtue of some holes that it has in its lower part. Pressurized air is simultaneously supplied from the air supply line and enters the container pushing the oil through the suction pipe, thus emptying the container and supplying oil through a discharge port that the device has on one of its sides. The supply of pressurized air is achieved through a container tank of this pressurized fluid, which generates maintenance costs of high pressure values not to mention the risk of explosion that can mean injecting air into a container with hydrocarbon.

In general, these types of hydrocarbon extraction arrangements have a hydrocarbon extraction assembly that moves up and down along the length of the well so as to extract hydrocarbon from the bottom of the well. The extraction assembly is usually comprised of a collecting container, which is filled with hydrocarbon, and a suction tube welded to the latter and through which the hydrocarbon contained in the container passes and is sucked up by a pump. As it has been seen, the aspiration or suction tube is welded to the collector tube, generating welded joints that, being constantly subject to efforts and pressures, generate critical points that can put the general structure of the extraction arrangement at risk. In turn, the suction or aspiration tube usually has a discharge hole at an upper end that must remain inside a sealed chamber in order to proceed with the extraction or discharge of the hydrocarbon by activating the pump. The fact of having seals, airtight chambers and welds, makes it necessary to operate with certain precautions to avoid possible accidents, as well as causes both this type of structure and the maintenance tasks to increase associated costs.

That is why it would be very appropriate to have a new arrangement that reduces the number of welds, security seals, joints, gaskets or watertight chambers, among others, in order to reduce risks, costs, facilitate maintenance and provide practicality during extraction.

BRIEF DESCRIPTION OF THE INVENTION

It is therefore an object of the present invention to provide a hydrocarbon extraction arrangement in oil wells, which allows reducing the number of parts such as joints, seals, gaskets and sealed chambers, among others, in order to facilitate both the tasks of maintenance as well as operation.

It is yet another object of the present invention to provide an extraction arrangement having at least one suction tube in a fixed position in the wellhead so that only one collecting container is lowered and raised.

It is also another object of the present invention to provide an extraction arrangement that reduces the related costs, as well as allows simplifying and optimizing the physical space of the place.

It is yet another object of the present invention to provide a hydrocarbon extraction arrangement in oil wells, wherein the arrangement has a wellhead connected to a well casing, and an extraction assembly operatively arranged within said well casing, said extraction assembly being formed by a fluid collecting container having an open upper end for the entry of oil into it and a closed lower end, and the extraction assembly is connected to a traction cable for raising and lowering it and, wherein the arrangement comprising: at least one suction tube arranged in a fixed position in said wellhead and having at least one end portion positioned within said well casing with at least one suction inlet operatively submersible in the hydrocarbon extracted and contained within said collecting container, and an opposite end connected to a suction pump, said suction tube being radially offset with respect to the geometric center of the casing and remaining operatively inside the collecting container when the latter is in a hydrocarbon discharge position.

BRIEF DESCRIPTION OF THE DRAWINGS

For greater clarity and understanding of the object of the present invention, it has been illustrated in several figures, in which the invention has been represented in one of the preferred embodiments, all by way of example, wherein:

FIG. 1 shows a schematic and sectional view of a hydrocarbon extraction arrangement according to the prior art, together with a surface actuator and wellhead;

FIGS. 2 a to 2 c show a schematic and sectional view of the oil lowering, raising and extraction stages by means of a hydrocarbon extraction arrangement according to the prior art;

FIG. 3 shows a schematic and sectional view of the hydrocarbon extraction arrangement according to the present invention, wherein a collector tube can be seen in the lowered operating position, and alternative means for centralizing fluid collecting container;

FIG. 4 shows a schematic and sectional view of the hydrocarbon extraction arrangement according to the present invention, wherein the collector tube with liquid hydrocarbon can be seen in the raised operating position;

FIG. 5 shows a schematic and sectional view of the hydrocarbon extraction arrangement according to the present invention, wherein the collector tube can be seen in position for the extraction of the liquid hydrocarbon contained inside it by means of a suction tube in a fixed position; and

FIG. 6 shows a partial sectional view of an alternative embodiment of a actuating bushing in accordance with the present invention; and

FIG. 7 shows an enlarged view of an end portion of the suction tube according to the present invention, in which a means of sealing the end of said suction tube and another alternative of a means for centralizing the fluid collecting container is illustrated.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures, it can be seen that the invention consists of a new extractor arrangement for the extraction of hydrocarbons that allows related costs to be reduced by simplifying the structure, reducing the number of parts used in this type of arrangement, and consequently reducing associated risks and facilitating both operation and maintenance tasks. It is emphasized that, for the case of the present invention, reference numbers from 100 will be used.

The arrangement of the present invention is placed in the surface of an oil well. Where, along the depth of said well, a casing or jacket 101 is housed, which reaches a depth such that it comes into contact with the hydrocarbons to be extracted. Generally, said hydrocarbons are present in a liquid state with solid particles and in a gaseous state at a certain pressure. As explained in relation to the prior art, there are often cases in which oil wells do not have the necessary internal pressure for the fluid to flow easily to the surface without the help of pumping elements. These wells, which may be somewhat depleted or semi-empty, require devices and pumping systems to be able to extract the remaining oil.

According to FIGS. 3 to 5 , the extraction arrangement of the present invention is indicated by the general reference 102 and has a wellhead 103 arranged on the surface of a well. A spool or surface actuator (not shown) mounted in the proximity of said wellhead 103 is provided and, coiled thereon, a traction cable 104 is arranged which is connected to the upper end of an extraction assembly 105 by means of which extraction of oil will be carried out. It is emphasized that said spool or surface actuator is sufficiently known in the field of art, and for such reasons it has not been illustrated. Likewise, said extraction assembly 105 is moved vertically inside the well, reaching a depth such that it comes into contact with the oil and submerges within it, and then rises and thus allows the extraction of the hydrocarbon or oil.

For its part, said well head 103 has a head support 106 inside which a press tee assembly 107 is provided which comprises an extension tube 108 whose lower end is connected to a safety valve 109, while its upper end is connected to a gland stop 110. The safety valve 109 is connected to a flange 111, while internally between said extension tube 108 and said gland stop 110 there is a plurality of conical rubber seals 112 which allow cleaning the cable and in turn prevent gas leakage. Said traction cable 104 passes (up and down) through said conical rubber seals 112. The arrangement of conical seals 112 will allow the automatic cleaning of the traction cable 104 when it passes through them and, in turn, will prevent gas leakage as mentioned above. Likewise, said traction cable 104 passes guided through a bronze guide bushing 113 and has, in part of its stroke, at least one sensor actuating bushing 114 which is detected by means of a proximity sensor 115 arranged in a part of said tee press assembly 107. It is highlighted that said proximity sensor 115 can be a Namur sensor, while to avoid possible gas leaks, the present invention provides a guide 116 for the traction cable that can be made of PVC and that is arranged upstream said gland stop 110 together or not, with said guide bushing 113.

Alternatively, the sensor actuating bushing is not attached to the traction cable 104. Instead, an actuating bushing 128 is provided which rests on a support 129 at a close distance from the proximity sensor 115. In this alternative and according to FIG. 6 , it can be seen that the actuating bushing 128 has an internal through passage 130 through which the traction cable 104 passes, lowering and raising a secondary bushing 131 being attached to it. When the traction cable 104 is raised so that hydrocarbon discharge is carried out, the secondary bushing 131 comes into contact with the actuating bushing 128 so as to drag it and rise together so as to allow the actuating bushing 128 to be sensed by the proximity sensor 115 and sending the corresponding signal to start with the hydrocarbon suction or discharge. When lowering the traction cable 104, the actuating bushing 128 comes into contact again with said support 129 (due to their structural configurations) being retained and remaining seated on it, causing the secondary bushing 131 to be released and continue its movement in conjunction with the traction cable 104.

It is clarified that the support 129 has a central opening 132 such that its diameter allows the passage of the secondary bushing 131 attached to the traction cable, but not the passage of the actuating bushing 128 in order to retain it and prevent it from continuing to descend. Likewise, the diameter of both the internal passage 130 of the actuating bushing 128 as well as the external passage of the secondary bushing 131 will be such that they allow the actuating bushing 128 to be driven through the secondary bushing 131. That is, the diameter of the internal passage 130 of the actuating bushing 128 can be slightly smaller than the external diameter of the secondary bushing 131 in order to allow the passage of the traction cable 104 freely but generating a stop or contact against it and thus allow the actuating bushing 128 to be pulled upwards. As well, the inverse case may occur in which the diameter of the internal passage 130 of the actuating bushing 128 is slightly greater than the external diameter of the secondary bushing 131, causing the latter to enter its interior, generating a coupling joint between parts and thus allowing driving the actuating bushing 128 upwards.

In both cases, the same operating phenomenon will occur when the traction cable is lowered, since the actuating bushing 128 abuts against the support 129 being retained in it, and the secondary bushing 131 is released, which continues to descend together with the traction cable 104. Although it has not been illustrated or referenced, in the event that the secondary bushing 131 fits into the internal passage 130 of the actuating bushing 128 because the diameter of the internal passage 130 is slightly larger than the external diameter of the secondary bushing 131, coupling elements or means can be provided that facilitate the coupling between the parts in order to help reduce both the impacts and the frictions generated in them.

On the other hand, and referring again to the extraction assembly 105, it is formed by a fluid collecting container 117 that has an open upper end 118 for the entry of oil into it and a lower end 119 closed by a ball valve 120. It is emphasized that the constructive features of the collecting container mentioned above are not limiting for the invention, since other types of collecting containers can be considered and used without any inconvenience.

With regard to one of the notable novelties of the present invention, the fluid collecting container 117 is independent of a suction tube 121, unlike the conventional extraction arrangements of the prior art known up to now, which have the suction tube welded or joined to the collecting container by means of welds, joints, seals, etc., and that has high associated costs.

According to FIGS. 3 to 5 , said suction tube 121 is arranged in a fixed position in said well head 103 and has at least one end portion 122 positioned inside said well casing 104, and downstream the safety valve 109. Said suction tube 121 and more particularly said end portion 122 comprises at least one suction inlet 123 operatively submersible in the hydrocarbon extracted and contained within said collecting container 117 when the latter is in the discharge position. Likewise, said suction tube 121 comprises an end 124 opposite to the first one connected to a suction pump 125 through which suction and extraction of the liquid hydrocarbon contained within said collecting container 117 will be carried out. The suction tube 122 will have the necessary length to extract the greatest amount of hydrocarbon collected inside the collector 117, thus being able to have the same or less length than the collector 117.

For design reasons and in order to avoid possible accidents, the suction tube 121 and more particularly the end portion 122 is positioned fixed and radially offset with respect to the geometric center of the casing 101 so as to avoid any type of contact with the traction cable 104 and that this can cause possible inconveniences. As will be described below, the end portion 122 of the suction tube 121 is operatively inside the collecting container 117 when the latter is in a hydrocarbon discharge position (FIG. 5 ). It is emphasized that, in order to facilitate the centering of the collecting container 117 when it is rising and close to the end portion 122 of the suction tube 121 and thus allow the end portion 122 of the suction tube 121 to be available inside it, a centralizer 126 is provided in the upper part of said collecting container 117. The centralizer 126 is part of the same collector or a removable aggregate piece with frustoconical shape, which opens radially outwards to facilitate the entry of the suction tube 121 inside the collector 117 when it rises and reaches the lower end of the tube 121. The centralizer can be made of a different material than the collector 117, for example rubber, or another plastic material. According to FIG. 3 , and to further facilitate the centralization of the collecting container 117, a second centralizer 133 mounted on a portion of the collecting container 117 can alternatively be added. Said centralizer 133 will have a plurality of flexible centering arms that abut against the walls of the casing 101 in order to fix and secure the position of the collecting container inside it. It should be noted that the second centralizer 133 can be any type of pipe centralizer known in the field of the art.

With regard to the operating stages of the extraction arrangement of the present invention, when it is desired to fill the collecting container 117, FIG. 3 , the spool lowers it inside the well until it reaches the depth corresponding to the oil level at the bottom of the well, allowing it to submerge in the oil so that the latter enters the collecting container 117 through the lower end 119. As the lower end 119 comes into contact with the oil 127, the ball valve 120 rises allowing the entry of the oil inside the collecting container 117. Once the collecting container 117 is full, it is raised and the weight of the fluid contained therein pushes the ball valve 120 downwards, closing the fluid passage and retaining it therein. It is emphasized again that the invention is not limited to said type of collecting container, but other collecting containers can be considered and used without any inconvenience, the method or form of filling them being dependent on each type of collecting container. For example, the upper end of collector 117 may be fully open where the centralizer is located. Also by way of example, the centralizer 126 can be a separate accessory that is threaded into the open upper end, also threaded into that of the collector 117. In this case, a ball valve would not be necessary and the hydrocarbon would enter the collector 117 through its port or open upper end 118 when the collector is submerged within the liquid mass of oil. The collector shall have the sufficient weight or added weight to facilitate its sinking into the mass of oil when necessary.

Thus, the collecting container 117 is raised by means of the operation of the spool that pulls the traction cable 104 upwards, as illustrated in FIG. 4 . According to FIG. 5 , the collecting container 117 is raised until the proximity sensor 115 detects the sensor actuating bushing 114. In said position, the end portion 122 of the suction tube 121 is submerged in the oil 127 contained within the collecting container 117, and the suction inlet 123 is adjacent to the bottom or lower end 119 of the collecting container 117 so as to proceed with the extraction through the actuation of the suction pump 125. Upon detecting the actuating bushing 114, and the collecting container 117 being in said hydrocarbon discharge or extraction position, the Namur proximity sensor 115 sends the corresponding signal to said suction pump 125 so as to activate it and proceed with the extraction of the hydrocarbon—oil 127. Once the oil contained within the collecting container 117 has been extracted, it is lowered again and so on and so forth as many times as necessary.

On the other hand, and according to FIG. 7 , an alternative is provided for the centralization of the collecting container 117. Wherein, said alternative comprises at least one external guide 134 that extends parallel and adjacent to said suction tube 121, more particularly extending longitudinally in coincidence with the section or vertical portion of said tube 121 that is inside the well. Said external guide 134 is fixed to both end parts of the vertical section of the suction tube 121 by means of respective supports 135. Although only the fixing of the lower part of the external guide 134 to the suction tube 121 has been illustrated with its respective support 135, it is understood that the upper end of the external guide 134 that is fixed to the upper portion of the vertical section of the tube 121 (which is inside the well) will have the same support 135, thus allowing that said external guide 134 remains fixed and parallel to said portion or vertical section of the suction tube 121. Said external guide 134 can be any type of rod, cable, or the like.

Likewise, there is provided at least one spring hook 136 arranged in the proximity of the lower end of the external guide 134 and which houses inside it a lower portion of said external guide 134, said traction cable 104 passing through its interior. In this way, the spring hook 136 together with the external guide 134 will define a reduced and fixed space for the pendular movement of the traction cable 104 inside the well (when it goes up and down) and consequently they will restrict the movement that the collecting container 117 may be subject to, centralizing it in this way and reducing any margin of error when it is positioned so that the suction tube 121 enters it.

On the other hand, in FIG. 7 , a fluid retention means 137 is provided arranged in the end portion 122 of the suction tube 121 and in the proximity of the suction inlet 123, which comprises a ball valve 138 operatively housed in a seat 139, and an upper stop 140 that prevents the ball valve 138 from moving upwards inside the tube, beyond what is necessary, during suction and when the end of the tube is immersed in the fluid mass. The ball valve 138 will close the suction inlet 123 and allow suction tube to be filled with fluid, oil, or at least some amount of fluid inside and along the suction tube 121 each time discharge or emptying of the collecting container 117 is complete. If this variant of the invention is not available, although the system works very well, at each discharge stage the pump completely sucks all the fluid from the container, leaving the suction tube empty or with air. Said air in the next discharge stage has to be purged by the pump to then allow the suction of the fluid that comes from behind it. This purging can generate internal wear of the pump parts and in turn may not allow optimal performance of the pump.

By means of this alternative of the present invention, the suction pump 125 will perform the suction of the fluid contained within the collecting container 117, but at the end of its discharge, it will not suck until the suction tube 121 is completely emptied, but rather it will cut off its operation beforehand and keep a remnant of fluid inside it or all of it filled with fluid. This remnant or all of the fluid contained in the suction tube will prevent the presence of air or gas inside it, and consequently will prevent the suction pump 125 from having to prime it at each discharge stage. This functionality of having a remnant of fluid in the suction tube translates into a better performance of the pump and consequently in an optimization of the discharge process. Likewise, the internal parts of the pump are protected, extending its useful life. It should be noted that the ball valve 138 rises during the suction process carried out by the suction pump 125, allowing the passage of the fluid, and once the pump stops its operation, due to pressure differences, it falls until it settles in said seat 139, closing the suction inlet 123 and preventing the fluid inside the suction tube from leaking into the well. Likewise, due to the weight of the remaining fluid column contained within the suction tube, the ball is held in position and cannot rise, thus preventing gas or air from entering it from the well. On the other hand, at least one speed sensor 141 is provided which may be a capacitive sensor and which is arranged at a lower end portion of the suction tube 121. Said speed sensor 141 will detect the speed with which the collecting container 117 is rising in the proximity of the suction tube by means of the traction cable 104 and thus will send the corresponding signal to activate the suction pump 125. As mentioned, the activation or sending of the signal by means of said sensor 141 depends on the speed with which the traction cable 104 is rising to the collecting container 117 in the proximity of the suction tube 121 and which will be defined as activation speed “VA”, the activation speed being less than the speed of lowering or raising the collecting container.

That is, the collecting container 117 goes down the well at a given speed V′ until it meets the fluid at the bottom of the well. After the collecting container is filled, it rises at a predetermined speed V″. Said rise speed V″ is reduced as the collecting container 117 is in the proximity of the suction tube 121, more particularly the suction inlet 123 thereof. The speed V″ with which the collecting container rises decreases to a point where the speed sensor 141 interprets it as the activation speed “VA”, sending the corresponding signal so that the suction pump begins with the discharge or suction cycle. It should be remembered that the activation speed “VA” will be less than the descent speed V′ and rise speed V″. When detecting the speed VA, the sensor 141 understands that the suction inlet 123 is submerged in the fluid contained within the collecting container 117, sending the respective activation signal (by means of a PLC or similar) to the suction pump 125 to start the download process. The discharge is carried out until the emptying of the collecting container 117 is completed, but taking into account what was mentioned above in relation to leaving a remnant of fluid inside the suction tube to avoid the presence of air or gas and the consequent priming or purging of the pump. It is highlighted that the activation speed VA will depend on each installation and the operating parameters at that moment.

In this way, the present invention is constituted and constructed in such a way as to reduce the number of seals, joints, welds, sealed chambers and the like, and consequently reduce the costs of this type of structure, also providing a structurally much simpler extraction arrangement, that is easy to maintain, operate and involving lower related costs. Likewise, as there are no sealed chambers, seals and joints, maintenance costs and accident risks are reduced, making the extraction operation simpler and more efficient. 

I claim:
 1. An arrangement for extracting hydrocarbons in oil wells, wherein the arrangement has a wellhead connected to a well casing, and an extraction assembly operatively arranged within said well casing, said extraction assembly being formed by a fluid collecting container that has an open upper end for the entry of oil into the collecting container and a lower end that can be closed, the extraction assembly being connected to a traction cable to raise and lower the extraction assembly and, wherein the arrangement for extracting hydrocarbons comprises: at least one suction tube arranged in a fixed position in said wellhead and having at least one end portion positioned within said well casing with at least one suction inlet operatively submersible in hydrocarbon extracted and contained within said collecting container, and an opposite end connected to a suction pump, said suction tube being radially offset with respect to a geometric center of the casing and remaining operatively inside the collecting container when the collecting container is in a hydrocarbon discharge position.
 2. An arrangement according to claim 1, wherein said collecting container has at least one centralizer at the open upper end of the collecting container.
 3. An arrangement according to claim 2, wherein said centralizer is a frustoconical ring that forms part of the upper end of the collecting container.
 4. An arrangement according to claim 2, wherein said centralizer is a frustoconical ring that is removably connected to the upper end of the collecting container.
 5. An arrangement according to claim 1, wherein said well head is provided with a press tee assembly comprising at least one proximity sensor capable of detecting an actuating bushing.
 6. An arrangement according to claim 5, wherein said proximity sensor for detecting the actuating bushing is attached to said traction cable.
 7. An arrangement according to claim 5, wherein said actuating bushing rests on a support in the proximity of the proximity sensor and comprises an internal through passage through which said traction cable passes, which is provided with a secondary bushing being attached to it and which comes into operative contact against said actuating bushing so as to drag it up and/or down according to the movement of the traction cable.
 8. An arrangement according to claim 5, wherein said wellhead further comprises a gland stop arranged upstream of said press tee assembly, which comprises a plurality of conical seals through which said traction cable rises and falls, wherein at least one cable guide and guide bushing are in the upper part of said gland stop.
 9. An arrangement according to claim 8, wherein said conical seals are made of rubber.
 10. An arrangement according to claim 8, wherein said cable guide is made of PVC.
 11. An arrangement according to claim 8, wherein said guide bushing is made of bronze.
 12. An arrangement according to claim 5, wherein said proximity sensor is a Namur sensor.
 13. An arrangement according to claim 1, wherein a centralizer is further provided which comprises: at least one external guide that extends parallel and adjacent to a vertical section of the suction tube positioned inside the casing, said external guide being fixed on both end portions of the vertical section of the suction tube by means of respective supports; and at least one spring hook arranged in the proximity of a lower end of the external guide and housing inside it a lower portion of said external guide, said traction cable passing through the at least one spring hook.
 14. An arrangement according to claim 1, wherein at least one fluid retaining means is provided arranged in the end portion of the suction tube and in the proximity of the suction inlet, the at least one fluid retaining means comprising a ball valve operatively housed on a seat, and an upper stop.
 15. An arrangement according to claim 1, wherein a capacitive speed sensor is provided in a portion of said suction tube. 